1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the zoom lens system, which are suitable for an image pickup apparatus using a solid-state image pickup element, such as a video camera, an electronic still camera, a broadcasting camera or a monitor camera, or an image pickup apparatus such as a silver-halide film camera.
2. Description of the Related Art
In recent years, an image pickup apparatus using a solid-state image pickup element, such as a video camera, a digital still camera, a broadcasting camera, or a monitor camera, as well as a silver-halide film camera has high performance, and the entire apparatus is downsized. Then, a photographing optical system used for the image pickup apparatus is required to be a high-resolution zoom lens system having a short total lens length, a compact (small) size, and a high zoom ratio (high magnification-varying ratio). As a zoom lens system which meets the requirements, there is known a positive-lead type zoom lens system having a lens unit of positive refractive power disposed on the object side. As the positive-lead type zoom lens system, there is known a zoom lens system constituted of five lens units having positive, negative, positive, negative, and positive refractive powers arranged in the stated order from the object side to the image side (see Japanese Patent Application Laid-Open No. 2003-287681 and U.S. Pat. No. 7,679,837).
Generally, a zoom lens system of a small size of the entire system and a high zoom ratio can be realized by enhancing refractive power of a main magnification-varying lens unit and increasing a movement amount of the main magnification-varying lens unit in zooming. However, when the refractive power of the main magnification-varying lens unit is enhanced and the movement amount of the main magnification-varying lens unit is increased, a high zoom ratio can be realized easily, but aberration variation in zooming is increased so that it is difficult to obtain high optical performance over the entire zoom range.
In the five-unit zoom lens system described above, in order to realize a high zoom ratio, a small size of the entire lens system, and good optical performance, it is important to set appropriately refractive power of each lens unit, a move condition or the like of each lens unit in zooming. In particular, it is important to set appropriately move conditions of third and fourth lens units in zooming and a refractive power (inverse number of a focal length) of the second lens unit. If these structures are not set appropriately, it is significantly difficult to realize a small front lens effective diameter, and to obtain high optical performance over the entire zoom range whereas retaining a high zoom ratio.